Robotic tools for space manipulators fall into one of two categories: 1) tools that are used to operate upon prepared interfaces (i.e. hardware that was designed together with the tools themselves, to facilitate the execution of robotic operations), and 2) tools that are used to operate upon unprepared interfaces (i.e. hardware that was not specifically designed to accommodate robotic operations, and that may be designed in such a way that make robotic operations very difficult).
The Special Purpose Dextrous Manipulator, or Dextre, provided to the International Space Station (ISS) by the Canadian Space Agency (CSA), is equipped with tools targeted to prepared interfaces. Dextre's tools are described at the following link: http://www.asc-csa.gc.ca/eng/iss/dextre/toolbox.asp. They are described in greater detail below.
a) Socket Extension Tool (SET). Dextre grips this tool with its end-effector, and uses it to extend the reach of the end-effector's socket driver mechanism. It can actuate 7/16″ bare bolts. This type of bolt is a standard size used for tie-down interfaces on the ISS. The SET incorporates a wobble socket, which provides the necessary compliance for robotically interfacing to a tie-down bolt with no co-located visual target. This tool was designed by MDA.
b) Robot Micro Conical Tool (RMCT). Dextre's end-effector was designed to directly interface with Micro-Fixtures and H-Fixture (robotic grasp features with a square/rectangular profile). Another fixture commonly used on the ISS is the Micro Conical Fitting (MCF) which, although designed to be robotically compatible with a collocated visual target, possesses round profiles and therefore is not directly compatible with Dextre's end-effector jaws. The RMCT is a tool equipped with a micro-fixture that allows Dextre to grasp it. The RMCT then allows Dextre to pick up payloads that are equipped with an MCF. This tool was designed by Oceaneering Space Systems.
c) Robotic Offset Tool. Dextre grips this tool with its end-effector, and uses it to access secondary tie-down bolts on specific station payloads (Orbit Replaceable Units or ORUs) where clearance constraints with adjacent equipment prevents Dextre's end-effector from being able to engage its socket drive directly to the tie down bolt. The tool gives Dextre access to tie-down points that it would be unable to access otherwise. The tool is designed for compatibility with Dextre, and with specific ISS payloads. This tool was designed by Boeing.
In addition to tools designed for ISS payloads, the Hubble Robotic Servicing Mission (http://www.edcheung.com/job/hrsdm/hrsdm.htm) explored the development of Dextre compatible tools that could be used to service hardware that had been launched on the Hubble Space Telescope. The Hubble Space Telescope was designed for servicing by astronauts—it is not equipped with features to facilitate robotic servicing, such as grapple fixtures, visual cues, or a physical equipment layout that provides a generous robotic workspace envelope. A ground testbed version of Dextre was installed at Goddard Space Flight Center, and demonstrations of tool concept prototypes were performed on a full scale mockup of the Hubble telescope. Robotic tools developed for these unprepared interfaces included electrical connector tools and tools that were used to access and actuate door latch fasteners. Hubble interface designs made no accommodations for robotic operations—special tools had to be designed for these unprepared interfaces that allowed operators to perform operations remotely and reliably.
The Hubble Servicing Mission was eventually cancelled, but the development of tools for unprepared interfaces continued both at Goddard Space Flight Center and at MDA/CSA.
Other prepared, robotically compatible tool interfaces have been developed by the European Space Agency (ESA). The Compact Tool Exchange Device (CTED) is designed for Eurobot, a three arm robot concept that is being developed to perform extravehicular activities (EVA) on the ISS. A description of this interface can be found at the following link and paper: http://www.esa.int/TEC/Robotics/SEMRIQNSP3F_0.html, A novel concept for a tool exchange device, Kester, G. J. A. N.; Visser, Proceedings of the 11th European Space Mechanisms and Tribology Symposium, ESMATS 2005, 21-23 Sep. 2005, Lucerne, Switzerland). CTED will enable the exchange of end effectors or tools, while allowing control signals and electrical power to pass from the arm to the tool. It consists of two types of components, one active unit, fixed to the robot arm, and several passive parts, fixed to the different tools and end effectors. Once the tool is positioned within reach of the arm, CTED is intended to automatically perform the attachment and release of the tool and the mating and de-mating of its electrical connectors. CTED provides alignment features that help guide the robot arm into the correct position and orientation for latching.
NASA has proposed a Robotic Refueling Mission (RRM) which is an external International Space Station experiment which is designed to demonstrate and test tools and methodologies required to refuel satellites in space, see Nasa Facts, article entitled “Robotic Refueling Mission” (FS-2011-3-11-GSFC (rev June 25)) (www.nasagov). This publication refers to tools to be tested including a Wire Cutter tool, Blanket Manipulation Tool, Multifunction Tool, the Safety Cap Removal Tool, and the Nozzle Tool. More details of these tools can be found at http://www.nasaspaceflight.com/2011/07/sts-135-enabling-new-era-robotic-satellite-refuelling-space/
Examining all of the existing robotic designs for handling multiple types of tools, a common feature is the use of a general robotic end-effector or hand which is capable of holding a tool which has its own source of motive power to apply force or torque. This is illustrated by the OTCM and CTED above. They are capable of grasping the tool and passing power to the motor(s) which provide actuation within the tool. However this means that for each tool held by the end-effector, it must possess its own single or multiple actuator. If the servicing mission requires a large number of powered tools, this will result in a large number of actuators being required in the overall robotic system to be capable of performing a variety of servicing functions. Generally, actuators are also required for grasping different tools and for adjusting the orientation of these tools.
Actuators add mass and complexity to the robotic device, and reduce robustness. Each additional actuator requires power, and necessitates the inclusion of redundancy schemes. As such, each additional actuator added to an end-effector increases the mass of that end-effector, and due to the need for more power and redundancy schemes, the mass increase is generally larger than the mass of the actuator itself. Additional mass added to the robot decreases the payload capacity of the robot, and in the case of space robotics, increases the cost of the overall mission.